LMS
Basic Concepts
Lite Memory Sanitizer (LMS) is a tool used to detect memory errors on a real-time basis. LMS can detect buffer overflow, Use-After-Free (UAF), and double free errors in real time, and notify the operating system immediately. Together with locating methods such as Backtrace, LMS can locate the code line that causes the memory error. It greatly improves the efficiency of locating memory errors.
The LMS module of the OpenHarmony LiteOS-M kernel provides the following functions:
- Supports check of multiple memory pools.
- Checks the memory allocated by LOS_MemAlloc, LOS_MemAllocAlign, and LOS_MemRealloc.
- Checks the memory when bounds-checking functions are called (enabled by default).
- Checks the memory when libc frequently accessed functions, including memset, memcpy, memmove, strcat, strcpy, strncat and strncpy, are called.
Working Principles
LMS uses shadow memory mapping to mark the system memory state. There are three states: Accessible, RedZone, and Freed. The shadow memory is located in the tail of the memory pool.
- After memory is allocated from the heap, the shadow memory in the data area is set to the Accessible state, and the shadow memory in the head node area is set to the RedZone state.
- When memory is released from the heap, the shadow memory of the released memory is set to the Freed state.
- During code compilation, a function is inserted before the read/write instructions in the code to check the address validity. The tool checks the state value of the shadow memory that accesses the memory. If the shadow memory is in the RedZone statue, an overflow error will be reported. If the shadow memory is in the Freed state, a UAF error will be reported.
- When memory is released, the tool checks the state value of the shadow memory at the released address. If the shadow memory is in the RedZone state, a double free error will be reported.
Available APIs
The LMS module of the OpenHarmony LiteOS-M kernel provides the following APIs. For more details about the APIs, see the API reference.
Table 1 LMS module APIs
Development Guidelines
How to Develop
The typical process for enabling LMS is as follows:
-
Configure the macros related to the LMS module.
Configure the LMS macro LOSCFG_KERNEL_LMS, which is disabled by default. Run the make update_config command in the kernel/liteos_m directory, choose Kernel, and set Enable Lite Memory Sanitizer to Yes.
-
Modify the compile script of the target module.
Add "-fsanitize=kernel-address" to insert memory access checks, and add the -O0 option to disable optimization performed by the compiler.
The modifications vary depending on the compiler (GCC or Clang) used. The following is an example:
if ("$ohos_build_compiler_specified" == "gcc") { cflags_c = [ "-O0", "-fsanitize=kernel-address", ] } else { cflags_c = [ "-O0", "-fsanitize=kernel-address", "-mllvm", "-asan-instrumentation-with-call-threshold=0", "-mllvm", "-asan-stack=0", "-mllvm", "-asan-globals=0", ] }
-
Recompile the code and check the serial port output. The memory problem detected will be displayed.
Development Example
This example implements the following:
- Create a task for LMS.
- Construct a buffer overflow error and a UAF error.
- Add "-fsanitize=kernel-address", execute the compilation, and check the output.
Sample Code
The code is as follows:
#define PAGE_SIZE (0x1000U)
#define INDEX_MAX 20
UINT32 g_lmsTestTaskId;
char g_testLmsPool[2 * PAGE_SIZE];
STATIC VOID testPoolInit(void)
{
UINT32 ret = LOS_MemInit(g_testLmsPool, 2 * PAGE_SIZE);
if (ret != 0) {
PRINT_ERR("%s failed, ret = 0x%x\n", __FUNCTION__, ret);
return;
}
}
static VOID LmsTestOsmallocOverflow(VOID)
{
PRINTK("\n######%s start ######\n", __FUNCTION__);
UINT32 i;
CHAR *str = (CHAR *)LOS_MemAlloc(g_testLmsPool, INDEX_MAX);
PRINTK("str[%2d]=0x%2x ", INDEX_MAX, str[INDEX_MAX]); /* trigger heap overflow at str[INDEX_MAX] */
PRINTK("\n######%s stop ######\n", __FUNCTION__);
}
static VOID LmsTestUseAfterFree(VOID)
{
PRINTK("\n######%s start ######\n", __FUNCTION__);
UINT32 i;
CHAR *str = (CHAR *)LOS_MemAlloc(g_testLmsPool, INDEX_MAX);
LOS_MemFree(g_testLmsPool, str);
PRINTK("str[%2d]=0x%2x ", 0, str[0]); /* trigger use after free at str[0] */
PRINTK("\n######%s stop ######\n", __FUNCTION__);
}
VOID LmsTestCaseTask(VOID)
{
testPoolInit();
LmsTestOsmallocOverflow();
LmsTestUseAfterFree();
}
UINT32 Example_Lms_test(VOID){
UINT32 ret;
TSK_INIT_PARAM_S lmsTestTask;
/* Create a task for LMS. */
memset(&lmsTestTask, 0, sizeof(TSK_INIT_PARAM_S));
lmsTestTask.pfnTaskEntry = (TSK_ENTRY_FUNC)LmsTestCaseTask;
lmsTestTask.pcName = "TestLmsTsk"; /* Task name. */
lmsTestTask.uwStackSize = 0x800;
lmsTestTask.usTaskPrio = 5;
lmsTestTask.uwResved = LOS_TASK_STATUS_DETACHED;
ret = LOS_TaskCreate(&g_lmsTestTaskId, &lmsTestTask);
if(ret != LOS_OK){
PRINT_ERR("LmsTestTask create failed .\n");
return LOS_NOK;
}
return LOS_OK;
}
Verification
The output is as follows:
######LmsTestOsmallocOverflow start ######
[ERR]***** Kernel Address Sanitizer Error Detected Start *****
[ERR]Heap buffer overflow error detected
[ERR]Illegal READ address at: [0x4157a3c8]
[ERR]Shadow memory address: [0x4157be3c : 4] Shadow memory value: [2]
OsBackTrace fp = 0x402c0f88
runTask->taskName = LmsTestCaseTask
runTask->taskID = 2
*******backtrace begin*******
traceback fp fixed, trace using fp = 0x402c0fd0
traceback 0 -- lr = 0x400655a4 fp = 0x402c0ff8
traceback 1 -- lr = 0x40065754 fp = 0x402c1010
traceback 2 -- lr = 0x40044bd0 fp = 0x402c1038
traceback 3 -- lr = 0x40004e14 fp = 0xcacacaca
[LMS] Dump info around address [0x4157a3c8]:
[0x4157a3a0]: 00 00 00 00 00 00 00 00 | [0x4157be3a | 0]: 1 1
[0x4157a3a8]: ba dc cd ab 00 00 00 00 | [0x4157be3a | 4]: 2 2
[0x4157a3b0]: 20 00 00 80 00 00 00 00 | [0x4157be3b | 0]: 2 0
[0x4157a3b8]: 00 00 00 00 00 00 00 00 | [0x4157be3b | 4]: 0 0
[0x4157a3c0]: 00 00 00 00 00 00 00 00 | [0x4157be3c | 0]: 0 0
[0x4157a3c8]: [ba] dc cd ab a8 a3 57 41 | [0x4157be3c | 4]: [2] 2
[0x4157a3d0]: 2c 1a 00 00 00 00 00 00 | [0x4157be3d | 0]: 2 3
[0x4157a3d8]: 00 00 00 00 00 00 00 00 | [0x4157be3d | 4]: 3 3
[0x4157a3e0]: 00 00 00 00 00 00 00 00 | [0x4157be3e | 0]: 3 3
[0x4157a3e8]: 00 00 00 00 00 00 00 00 | [0x4157be3e | 4]: 3 3
[0x4157a3f0]: 00 00 00 00 00 00 00 00 | [0x4157be3f | 0]: 3 3
[ERR]***** Kernel Address Sanitizer Error Detected End *****
str[20]=0xffffffba
######LmsTestOsmallocOverflow stop ######
###### LmsTestUseAfterFree start ######
[ERR]***** Kernel Address Sanitizer Error Detected Start *****
[ERR]Use after free error detected
[ERR]Illegal READ address at: [0x4157a3d4]
[ERR]Shadow memory address: [0x4157be3d : 2] Shadow memory value: [3]
OsBackTrace fp = 0x402c0f90
runTask->taskName = LmsTestCaseTask
runTask->taskID = 2
*******backtrace begin*******
traceback fp fixed, trace using fp = 0x402c0fd8
traceback 0 -- lr = 0x40065680 fp = 0x402c0ff8
traceback 1 -- lr = 0x40065758 fp = 0x402c1010
traceback 2 -- lr = 0x40044bd0 fp = 0x402c1038
traceback 3 -- lr = 0x40004e14 fp = 0xcacacaca
[LMS] Dump info around address [0x4157a3d4]:
[0x4157a3a8]: ba dc cd ab 00 00 00 00 | [0x4157be3a | 4]: 2 2
[0x4157a3b0]: 20 00 00 80 00 00 00 00 | [0x4157be3b | 0]: 2 0
[0x4157a3b8]: 00 00 00 00 00 00 00 00 | [0x4157be3b | 4]: 0 0
[0x4157a3c0]: 00 00 00 00 00 00 00 00 | [0x4157be3c | 0]: 0 0
[0x4157a3c8]: ba dc cd ab a8 a3 57 41 | [0x4157be3c | 4]: 2 2
[0x4157a3d0]: 2c 1a 00 00 [00] 00 00 00 | [0x4157be3d | 0]: 2 [3]
[0x4157a3d8]: 00 00 00 00 00 00 00 00 | [0x4157be3d | 4]: 3 3
[0x4157a3e0]: 00 00 00 00 00 00 00 00 | [0x4157be3e | 0]: 3 3
[0x4157a3e8]: ba dc cd ab c8 a3 57 41 | [0x4157be3e | 4]: 2 2
[0x4157a3f0]: 0c 1a 00 00 00 00 00 00 | [0x4157be3f | 0]: 2 3
[0x4157a3f8]: 00 00 00 00 00 00 00 00 | [0x4157be3f | 4]: 3 3
[ERR]***** Kernel Address Sanitizer Error Detected End *****
str[ 0]=0x 0
######LmsTestUseAfterFree stop ######
The key output information is as follows:
-
Error type:
- Heap buffer overflow
- UAF
-
Incorrect operations:
- Illegal read
- Illegal write
- Illegal double free
-
Context:
- Task information (taskName and taskId)
- Backtrace
-
Memory information of the error addresses:
- Memory value and the value of the corresponding shadow memory
- Memory address: memory value|[shadow memory address|shadow memory byte offset]: shadow memory value
- Shadow memory value. 0 (Accessible), 3 (Freed), 2 (RedZone), and 1 (filled value)